I just want to be able to right-click somewhere and have the system bring up a a simple FTPS or SFTP daemon and have it try UPnP to open a firewall/NAT transversal. This daemon would spin up just for the transfer then shut itself down once done.

Even if both parties are behind a firewall that won't respect UPnP, there's UDP hole punching which should usually work. Mozilla would just need to host a server to handle the port number handoffs.

I imagine this would handle 90% of the use cases. The rest would use traditional 3rd party hosting, IM transfers, email, etc.